The present invention relates to the field of machining of hard metallic materials by cutting (e.g., shaping parts by removing excess material in the form of chips) with hard cutting tools, and more particularly to machining methods that reduce the thickness of a thermomechanically-affected layer (e.g., white layer) on as-machined surfaces of hard metal workpieces and/or mitigate the detrimental effects in machined surfaces of hard metal workplaces due to the thermomechanical load of a hard cutting tool machining the workpiece.
Specifically, the invention concerns machining of hard metallic parts, characterized by the surface hardness exceeding 42 Rockwell on Scale C, with hard cutting tools, characterized by the edge hardness exceeding 1500 Vickers. Machining of hard or hardened metallic parts brings about significant cost savings to the manufacturing industries through the reduction of heat-treating and machining steps in the production cycle and minimizing the extent of slow, finish-grinding operations. With the advent of hard, ceramic cutting tools and tool coatings, which include alumina (Al2O3), cubic boron nitride (CBN) and many other advanced materials, machining of hard metals has become feasible and includes outer diameter (OD) turning, inner diameter turning (boring), grooving, parting, facing, milling, drilling, and numerous other cutting operations.
A significant limitation of the widespread use of hard metal machining is the so-called “white layer” effect, a microscopic alteration of the as-machined surface of a workpiece, which effect is produced in response to an extremely high thermomechanical load exerted at the as-machined surface by the cutting tool. Although not fully understood, the thermomechanically-affected workpiece surface comprising an etching-resistant white layer is undesired because of associated tensile stresses, e.g., reduced fatigue-resistance, lower fracture toughness, and/or reduced wear resistance of parts produced. See, B. J. Griffins, White Layer Formation at Machined Surfaces and Their Relationship to White Layer Formations at Wom Surfaces, J. of Tribology, April 1985, Vol. 107/165.
It has been reported that a sharper and/or not worn cutting edge, as well as the conventional flooding of a cutting tool with a water-based, emulsified oil coolant, contribute to the reduction in the detrimental tensile stresses and white layer. W. Konig, M. Klinger, and R. Link, Machining Hard Materials with Geometrically Defined Cutting Edges—Field of Applications and Limitations, Annals of CIRP 1990, Vol. 57, pp. 61-64. Hard machining with conventional flood cooling has been reexamined but found to be ineffective. H. K. Tonshoff and H. G. Wobker, Potential and Limitations of Hard Turning, 1st Int. Machining and Grinding Conf., Sep. 12-14, 1995, Dearborn, Mich., SME Technical Paper MR95-215. Moreover, sharp-finished cutting edges easily fracture in the case of inexpensive, Al2O3-based tools, leaving expensive CBN tools as the only current option. It has been noted that the use of coolants in hard machining should be avoided since cooling accelerates the edge wear and shortens overall life of CBN tools used for finish-hardturning. T. J. Broskea, PCBN Tool Failure Mode Analysis, Intertech 2000, Vancouver B. C., Canada, Jul. 17-21, 2000. Numerous other publications and machining textbooks have indicated that the use of coolants with inexpensive Al2O3 tools brings about a rapid fracture. Using non-cooled CBN tools (dry turning), the effect of cutting speed on white layer thickness during hardturning of a popular hardened bearing steel 52100 has been examined. Y. K. Chou and C. J. Evans, Process Effects on White Layer Formation in Hard Turning, Trans. of NAMRI/SME, Vol. XXVI, 1998, pp. 117-122. Results showed that only relatively low cutting speeds, translating into reduced productivity rates, assure an acceptably thin white layer. Thus, the machining technology of today offers no solution for making hard, white layer-free parts quickly and at reduced costs.
It is desired to have an apparatus and a method which minimize the alteration of workpiece surfaces during hard machining, and more specifically, which eliminate or minimize tensile and/or fluctuating surface stresses and etch-resistant white layer (i.e., the detrimental effects of “white layer”).
It is further desired to have an apparatus and method which produce better parts having less of the detrimental effects of a thermomechanically-affected layer (e.g., “white layer”) and which do so faster, at lower costs, and with less expensive tools.